1. Field of the Invention
The present invention relates to a method and apparatus for optical measurement of the thickness of a hot glass body by means of optical dispersion or chromatic aberration without direct contact with it immediately after its manufacture while it is still in a hot state.
2. Related Art
Certain dimensions must be maintained during production of glass bodies. Generally these dimensions are specified by the customer and may not exceed or fall below certain predetermined tolerance limits.
Currently exact measurements of the glass thickness are frequently made at the end of the cooling path, because the glass surface can be damaged by an earlier measurement with mechanical sensors and because contactless measurement does not work with hot glass. Thus e.g. laser triangulation is ruled out, since air schlieren produced by the hot glass distorts the optical path. Ultrasonic measurements are unusable, since they require a coupling medium between the glass and the ultrasonic source. White light interferometry does not operate under the rough production conditions. Confocal microscopy coupled with an auto-focus system is very expensive costing several hundred thousand euros for each measurement direction.
However measurement of the glass thickness as early as possible is desired since the amount of faulty products produced is smaller, the earlier the faults on the production line are detected and eliminated.
That can lead to considerable advantages in production, since production for each product can last up to several hours, until the product has reached the end of the cooling path, the so-called “cold end”, and thus until the measured values, which are required for correction at the hot end, are available.